The present embodiments relate to apparatus that can adjust a heat transfer surface area during chilling or freezing processes.
Known freezing systems that are used in, for example, in transit refrigeration (ITR) include mechanical compression refrigeration driven by diesel fuel motors, bunkers filled with CO2 dry ice, or CO2 liquid that is vaporized through heat exchangers mounted inside a refrigerated space and then discharged to an exterior of the space. The air inside the refrigerated space is cooled by forced or natural convection over the surface of the heat exchanger for the mechanical compression refrigeration system, the dry ice bunker or the liquid CO2 heat exchanger. The air temperature inside the refrigerated space will usually be either 0° F. (−18° C.) for a frozen food product, or 34° F. (1° C.) for a chilled product.
Precise temperature control of the air in the space using a mechanical compression refrigeration system is difficult, due to a low temperature difference between the refrigerant temperature and the desired air temperature and thus, a limited heat transfer rate. In addition, for refrigeration systems installed in trailers, the trailer doors are frequently opened for deliveries providing frequent rapid increases in trailer heat load. Precise temperature control of the air in the space is difficult for dry ice bunker systems because the heat exchanger surface always remains at minus 109° F. (−78° C.), and once that temperature is reached the heat transfer cannot be reduced. Therefore, air temperature will drop below the desired set point. Failure to maintain proper temperature control in the space may cause the temperature to be reduced to a rate below that which is acceptable for the product to be transported, and thereby damage the product.
In order to compensate for the anticipated increase in heat load, air temperature within the space will frequently be reduced to a temperature that is lower than desirable for the product being transported. This makes food products especially susceptible to damage, and will therefore likely result in the system efficiency being lowered in order to obtain the proper temperature control for the space.
Known systems also have a cold surface at the heat exchanger which tends to become covered in frost that has been condensed from air external to the refrigerated space being permitted to come into the space (such as when trailer doors are opened to access the product), thereby causing variation in heat transfer rate and potential loss of temperature control for the space. It is desirable to eliminate the frost build up on the heat exchanger surface and provide for a more uniform and consistent temperature of the product and the refrigeration space.